This invention relates to a novel semiconductor die. More specifically, this invention relates to a test die for evaluating a semiconductor assembly machine or process, and methods for making the die and for performing such evaluations by utilizing the test die.
In the semiconductor industry, assembly equipment and processes are evaluated prior to production commercial in order to maximize yield. A wafer sort prober electrically tests the integrity of the integrated circuits fabricated on the die. A test die is used in an assembly machine and then the machine, the wafer sort prober, and the die configuration are evaluated based on results obtained using the test die.
To illustrate the assembly process and components, shown in FIG. 1 is a silicon wafer 10 which may be cut into a plurality of dies 12. In a test die, bond pads are fabricated on the die with or without integrated circuits to permit testing of the arrangement of the bond pads and the performance of the assembly and wafer sort machines.
In order to form a usable package, a die is put on the central portion of a lead frame or the base of a package housing with leads, and each of the leads is electrically connected to a bond pad via a bonding wire by means of an assembly machine. A single die ready to be used in products has a plurality of bond pads on its surface. The distance between the center points of two adjacent bond pads is defined as the bond pad pitch.
Since different dies may have different bond pad pitches, a test die should be suitable for use in evaluating the effectiveness of an assembly machine or wafer prober on the die's bond pad pitch and the electrical integrity of the bond formed by the machine. As shown in FIG. 2, a bond ball 14 formed at the end of a bonding wire 15 descends upon a bond pad 16 layered on top of a polysilicon layer 20 and is then welded to the bond pad by use of an assembly machine. When too much force is applied to the assembly pad by a bond ball during the bonding process, the die may undesirably crack and crater 22 results.
In the past, test dies have included a plurality of bond pads having a uniform bond pad pitch fabricated on a single wafer. However, the use of a wafer dedicated to a particular bond pad pitch is costly because only a portion of the wafer may actually be utilized. A user would have to purchase many wafers having different bond pad pitches in order to test and evaluate the assembly and wafer sort machines or production process for a plurality of bond pad pitches.
The bond pad pitch of an actual device must be optimized with respect to an assembly machine, to accommodate the bond pads at the corners of the die and those that are not at the corners. FIG. 3 illustrates the connection of, for example, a set of corner bond pads 30 connected to leads 32 via bonding wires 34, and center bond pads 36 connected to leads as shown. Note that the distance between adjacent bonding wires connected to the set of center bond pads 36 is greater than the distance between adjacent bonding wires connected to the corner bond pads 30. Since the die is so small, there is the possibility that the bonding wires at the corners will undesirably contact one another and produce a short circuit.
With prior art test dies, it is difficult to verify open circuits or short circuits between adjacent bonding wires, and no effective method of determining the presence of cratering is available. Additionally, no effective method for evaluating array bonding has been discovered.
The difficulties in the preceding are not intended to be exhaustive but rather are among many which may tend to reduce the usefulness of prior art test dies. Other noteworthy problems may also exist; however, those presented above should be sufficient to demonstrate that test dies appearing in the past will admit to worthwhile improvement.